void function error in texfrag-document with old version of sx.el

[TobiasZawada]

(void function nil) error in texfrag-document with pre-#336 versions of sx.el.

The pre-#336 version of sx.el does not have sx-question-mode-after-print-hook. It must use a function advice for sx-question-mode--print-question. But that cannot be applied buffer-locally.

Therefore, if one switches texfrag on in one buffer texfrag-sx-after-print is also called in other buffers where texfrag-mode is not properly setup. Especially texfrag-preview-region-function is nil.